1. Field of the Invention
This invention relates to a toner used in image forming processes for making electrostatic latent images into visible images, such as electrophotography, and a toner used in toner jet recording.
2. Related Background Art
In recent years, machinery making use of electrophotography has begun to be used in printers for computer data output, facsimile machines and so forth in addition to copying machines for copying original images. Accordingly, machines are severely sought to be more compact, more light-weight, more high-speed and more high-reliability, and have come to be constituted of simpler components in various aspects. As a result, it is demanded for toners to have higher performance.
In particular, in respect of energy saving and office space saving, machines such as printers are required to be made more compact. On that occasion, containers which hold toners therein are also necessarily required to be made compact, and a low-consumption toner capable of printing many sheets with a small quantity is required.
Japanese Patent Applications Laid-open No. H3-84558, No. H3-229268, No. H4-1766 and No. H4-102862 disclose that the shape of toner base particles is made close to spherical shape by production processes such as spray granulation, solution dissolution, and polymerization. These methods, however, all require large-scale equipment for the production of toners. This is undesirable in view of production efficiency, and also toners obtained have not achieved sufficient reduction of toner consumption at the time of printing.
Japanese Patent Applications Laid-open No. H2-87157, No. H10-97095, No. H11-149176 and No. H11-202557 disclose that toner base particles produced by pulverization are made to undergo thermal or mechanical impact to modify the shape and surface properties of the toner base particles. However, the modification of the particle shape of toner base particles by these methods can not be said to be sufficient in reducing toner consumption at the time of printing, and also has caused deterioration in developing performance in some cases.
With the achievement of high-speed development and energy saving in recent years, it is also demanded to provide toners that can materialize lower-temperature fixing performance.
The melt behaviour required as toners is (1) to have high melt performance at low temperature and (2) to have high releasability even at high temperature. It is desired to create toners having such properties.
Physical properties given as an index of melt characteristics includes melt viscosity. As characteristics of ideal melt viscosity, it is preferable (1) that the melt start temperature is low and (2) that melt viscosity is kept constant at an appropriate value up to high temperature. In printers making use of a heat-and-pressure fixing system, both the characteristics are important factors because the former (1) is important in order to achieve energy saving and shorten stand-by time for printing, and also in view of an influence on machine surroundings where the electrophotography making use of the heat-and-pressure fixing system is used, and the latter (2) is important in order that the releasability from the heating roller is sufficiently kept even at high temperature and prints are prevented from staining because of adhesion of unfixed toner to the heating roller (i.e., a phenomenon of offset).
Resins having superior low-temperature melting properties may include polyester resins, which, however, though having a low melt start temperature, may greatly lower melt viscosity at high temperature.
Japanese Patent Application Laid-open No. S57-208559 discloses a toner containing a polyester resin and an anti-offset agent. This toner tends to cause some problem in respect of fluidity and agglomerative properties. Also, the polyester resin is difficult to pulverize in a process involving the step of pulverization and is disadvantageous in respect of the productivity of toner base particles produced by pulverization.
On the other hand, resins having superior releasability at high temperature include vinyl resins. The vinyl resins have the properties of readily obtaining high releasability such that the temperature at which the melt viscosity begins to lower is relatively high, but have a relatively high melt start temperature. However, in order to realize good fixing properties, if providing the binder resin with a low molecular weight to lower the temperature at which the melt viscosity begins to lower, a low release effect may result. Even if a release agent is used in vinyl resins with a low molecular weight in order to achieve low-temperature fixing, the melted resins themselves have so low a viscosity as to make it difficult to exhibit the necessary release effect.
Japanese Patent Application Laid-open No. S56-116043 discloses a toner making use of a resin obtained by polymerizing a vinyl monomer in the presence of a reactive polyester resin and allowing the polymer to have a high molecular weight through cross-linking reaction, addition reaction and grafting reaction in the course of polymerization. Further, Japanese Patent No. 2962809 discloses resin compositions for toners which has a polyester resin and a vinyl copolymer.
Toners containing in their toner base particles the vinyl polymer or gel content obtained by such cross-linking reaction may be expected to be improved in anti-offset performance. However, where the vinyl polymer obtained by such cross-linking reaction is used as a toner raw material, a polymer with high viscoelasticity may undergo large shear force at the time of melt kneading in producing toner base particles. This may accelerate the cutting of polymer molecular chains to lower the melt viscosity of the binder resin, and hence lower the anti-offset performance of the toner at the time of heat-and-pressure fixing. Also, the generation of heat because of the cutting of polymer molecular chains may cause a rise in temperature of the polymer itself at the time of melt kneading to make it difficult to achieve good dispersion of components contained in the toner base particles.
Japanese Patent Application Laid-open No. H10-087837 and Japanese Patent No. 3118341 disclose toners in which molecular weight distribution controlled to have a peak in each of a low-molecular weight region and a high-molecular weight region is formed and which have as a binder resin a resin composition constituted of a carboxyl-group-containing vinyl resin and as a cross-linking agent a glycidyl-group-containing vinyl resin.
Although these toners exhibit superior effect on the improvement of anti-offset properties, when using such a cross-linked resin, the resin has a high viscosity at the time of melt kneading, tending to result in coarse particles in producing toner base particles. As a result, the toner making use of the resultant toner base particles tends to cause faulty images due to sleeve coat non-uniformity, and such a tendency is remarkable especially in image forming apparatus of a high-speed development system.
It has been long-awaited to provide a toner which can satisfactorily achieve space saving, high speed and energy saving in printers.